Scanner banding problem

[Marco B]

Hi all,

I have a Canon 9950F flatbed that I have used for a couple of years now. Not very intensively, but on regular basis.

I know the scanners totally sucks for scanning film, and consequently have mainly used it for scanning small analog prints at about 400 ppi max, which it does very acceptably to even good.

Now I recently started noticing some banding issues. The banding is parallel to that of the scan unit and the CCD therein, that is, at a 90 degree angle with the scan units path down the scanned surface.

It is very regular banding turning up maybe each 1-2 seconds during the scan process. In between these bands, the scanned pixel surface looks fine.

Since the banding is parallel to the CCD, I think we can exclude a CCD issue, as the area without bands show each pixel can still operate properly.

Now I have read before about the possibility of issues with the light source. Not unlikely here, because they are cold cathode, not LED, and won't have an indefinite life time.

However: I also see it with the much less used negative / positive scanning! And since there is a second light unit in the top of scanner exclusively used for that, I think it is unlike both lamp units would go bust at the same time :confused:

So, what can cause such banding?

There is one other small thing that keeps nagging me: Some time ago, my electricity meter was replaced with a computerized "digital" one. Before that, it was just a simple "mechanical" one with a rotating disk.

Could this potentially cause some unwanted interference / small instabilities in the voltage that might be responsible for the banding, or am I completely rambling here :confused:

I have the vague feeling that the onset of the issues might have coincided with the installation of that unit...

I have attached a couple of screenshots. Please note the issue is not always that apparent at first. It is most visible at sharp light/dark transitions, showing up as greenish and reddish pixels, but as you can see, the banding passes from left to right through the entire image.

To make it more clear, I have upped the saturation of one of these screenshots to 100% in PS, which makes the banding much more clear (but of course screws up the colors entirely, but that is not the point here)

Scanning direction is top to bottom in these images!

The light sepia toned image is the original scan, the ugly orange one the max saturated, and the B&W one is the Green channel of the scan, that shows the affected bands maybe most clearly.

 

[Marco B]

Last remark: there is no apparent issue with the transportation of the scan unit along its scan path. It at least sounds normal and steady.

 

[pellicle]

Hi

particularly with the middle and right attachment I'm inclined to suggest some amount of dust at the place where the scanner takes its reference from. It has the character of that to me.

if you have disassembled and cleaned the scanner (including the optical path) then perhaps it may be a bung CCD in the array.

I'd bet on the first, but only a couple of bucks

 

[Marco B]

I did disassemble and had a closer look at the optical path (well, just the mirror and the view of the scanners lens in there). Not much to see, dusted it out with the vacuum cleaner, although there was nearly no problem at all. Also, the pattern is far to regular and repeating for a random dust problem.

Anyway, guess what, no joke this: I still doubt if it is the real issue, but I DID get just today a letter of my electricity company stating they "had detected a possible defect / malfunction of my electricity meter" and "it would be replaced sometime soon"... I guess they mean a batch of these meters has been found problematic.

Although I still don't have a clue how it can effect the scan results, if at all, I will be eager to see what happens if the meter is replaced...

Marco

 

[pellicle]

Hi

I did disassemble and had a closer look at the optical path (well, just the mirror and the view of the scanners lens in there).

did it help?

Also, the pattern is far to regular and repeating for a random dust problem.

actually, a dust problem is exactly regular, The dust sits on the same place of the glass each time, especially if it is under the glass. As the linear head of the scanner moves along you get its effect appear all the way along the path.

Recall that the line of CCD elements is across the page and the line of elements is pulled along the page when scanning.

dust on the mirror displays as flare, for example

http://www.vad1.com/photo/dirty-scanner/

I use Epson scanners, but as I understand it the calibration area is here for the model of Canon you suggested:

you can try an experiment and put a piece of something in that area and see what effect it has on the slide that is under it in the direction of the scan head movement.

 

[Marco B]

actually, a dust problem is exactly regular, The dust sits on the same place of the glass each time, especially if it is under the glass. As the linear head of the scanner moves along you get its effect appear all the way along the path.

Recall that the line of CCD elements is across the page and the line of elements is pulled along the page when scanning.

Pellicle, I probably did not state it clear enough, but the banding is also "across" the page, that is, parallel to the CCD line... So what happens, each 1 second or so, a band shows up while scanning, while scanned pixels inbetween are OK, showing the CCD can still deliver proper results.

If dust really was the problem, the banding should have been at a 90 degree angle with what it is now...

An issue with the cold cathode light lamp might be more logical, but from the few examples I have seen here or elsewhere, I am not sure. It is just so regular and minor. Most examples of lamp related issues show more pronounced issues, and less regular due to flickering or so... In addition, both scanning the platen AND scanning negatives show issues, and these use 2 DIFFERENT lamps (one as part of the moving scan unit, the other in the top lid of the scanner). I think it is highly unlikely both lamps show the same issue creeping up at the same time, unless both would be connected to the same high voltage unit, which they probably aren't, since that would require a high voltage connection from the unit to the lid, and there isn't one...

I will wait and see what happens when the electricity meter is replaced and report back than here on Hybridphoto.